Well the trick is kinda simple to be honest, you just copy pasta the following link into your browser, hit it and there you go.

Simply click on it:
steam://install/92 [http//steam]

Codename Gordon is not the only game that you can add to your steam account that way. There are multiple more examples out there.

Like this reddit thread here that includes some more unlisted games.

But I am not sure how ok valve is with such loopholes, so caution do it at your own risk.

This guide is just meant to be a short summary of how I got the game Codename Gordon. Like I wrote before I still do not know how valve could deal with such loopholes or this one in the future so do this at your own risk, it might be a banable offense and be against the rules and guidelines.

I also do not know if this might have consequences in the future, but that would be a problem to think of when the time is right.

And yes, I am surely not the first guy writing a guide about this.

-cheers